Methods of preparing gypsum wallboards containing styrene butadiene latex additive

ABSTRACT

Gypsum wallboard compositions are disclosed which contain functionalized styrene butadiene latex polymers. Methods for the use of the compositions in the manufacture of wallboard panels and sheets is presented.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/965,306, filed Sep. 27, 2001, now U.S. Pat. No. 6,525,116 which was acontinuation-in-part of U.S. patent application Ser. No. 09/237,512,filed Jan. 26, 1999, issued as U.S. Pat. No. 6,184,287 on Feb. 6, 2001.

FIELD OF THE INVENTION

The present invention relates generally to the production of low weight,high strength gypsum-containing compositions and panels. Moreparticularly, the present invention is directed to the production anduse of gypsum wallboard compositions and panels containingfunctionalized styrene butadiene latex polymers.

BACKGROUND OF THE INVENTION

A common method of constructing walls and ceilings includes the use ofinorganic wallboard panels or sheets, such as gypsum wallboard, oftenreferred to simply as “wallboard” or “drywall.” Wallboard can beformulated for interior, exterior, and wet applications. The use ofwallboard, as opposed to conventional walls made from wet plastermethods, is desirable because the installation of wallboard isordinarily less costly and less cumbersome when compared with theinstallation of conventional plaster walls.

Walls and ceilings made with gypsum wallboard panels typically areconstructed by securing, e.g., with nails or screws, the wallboardpanels to structural members, such as vertically-andhorizontally-oriented pieces of steel or wood often referred to as“studs.” When forming a wall from wallboard panels there will generallybe “joints” between adjacent wallboard panels because wallboard istypically supplied in standard-sized sheets. In most wallboardconstruction, the joints are cosmetically treated with reinforcing tapeand an adhesive material called “joint compound” so that the wall willhave a smooth finish similar to that obtained with conventional plasterwalls.

Generally, wallboard is produced by enclosing a core of an aqueousslurry containing calcined gypsum and other materials between two largesheets of wallboard cover paper. Various types of cover paper are knownin the art. After the gypsum slurry has set (i.e., reacted with thewater present in the aqueous slurry) and dried, the formed sheet is cutinto standard sizes. Methods for the production of gypsum wallboardgenerally are described, for example, by Michelsen, T. “BuildingMaterials (Survey),” Kirk-Othmer Encyclopedia of Chemical Technology,(1992 4^(th) ed.), vol. 4, pp. 618-619, the disclosure of which ishereby incorporated herein by reference.

A major ingredient of the gypsum wallboard core is calcium sulfatehemihydrate, commonly referred to as “calcined gypsum,” “stucco,” or“plaster of Paris.” Stucco has a number of desirable physical propertiesincluding, but not limited to, its fire resistance, thermal andhydrometric dimensional stability, compressive strength, and neutral pH.

Typically, stucco is prepared by drying, grinding, and calcining naturalgypsum rock (i.e., calcium sulfate dihydrate). The drying step in themanufacturer of stucco includes passing crude gypsum rock through arotary kiln to remove any free moisture (i.e., water which is notchemically bound) present in the rock.

The dried rock is then passed through a roller mill (or impact mill typeof pulverizer), wherein the rock is ground or comminuted to a desiredfineness. The dried, fine-ground gypsum can be referred to as “landplaster.” The land plaster is used as feed in calcination processes forconversion to stucco.

The calcination (or dehydration) step in the manufacture of stucco isperformed by heating the land plaster to liberate a portion of thechemically bound water molecules. Calcination of stucco can generally bedescribed by the following chemical equation which shows that heatingcalcium sulfate dihydrate yields calcium sulfate hemihydrate (stucco)and water vapor:CaSO₄.2H₂O+heat→CaSO₄.½H₂O+1½H₂O.

This calcination process step is performed in a “calciner,” of whichthere are several types known by those of skill in the art.

Uncalcined calcium sulfate (i.e., land plaster) is the “stable” form ofgypsum. However, calcined gypsum, or stucco, has the desirable propertyof being chemically reactive with water, and will “set” rather quicklywhen the two are mixed together. The setting reaction is a reversal ofthe above-described chemical reaction that occurs during the calcinationstep. Accordingly, the setting reaction proceeds according to thefollowing chemical equation, which shows that calcium sulfatehemihydrate is rehydrated to its dihydrate state upon the addition ofsufficient water:CaSO₄.½H₂O+1½H₂O→CaSO₄.2H₂O+heat.

The actual time required to complete the setting reaction generallydepends upon the type of calciner and the type of gypsum rock that isused to produce the gypsum, and can be controlled to some extent byusing additives such as, for example, retarders, set accelerators,and/or stabilizers. Generally, the time required for rehydration can beas little as about two minutes to as long as about eight hours,depending on the quantity of retarders, set accelerators, and/orstabilizers present.

Gypsum wallboard is generally manufactured utilizing commercialprocesses that are capable of operation under continuous, high-speedconditions. A conventional process for manufacturing the corecomposition of gypsum wallboard initially includes the premixing of dryingredients in a high-speed mixing apparatus. The dry ingredients caninclude calcium sulfate hemihydrate (stucco), an accelerator, and anantidesiccant (e.g., starch).

The dry ingredients are typically mixed together with a “wet” (aqueous)portion of the core composition in a pin mixer apparatus. The wetportion can include a first component, commonly referred to as a “paperpulp solution,” that includes a mixture of water, paper pulp, and,optionally, one or more fluidity-increasing agents, and a set retarder.The paper pulp solution provides a major portion of the water that formsthe gypsum slurry of the core composition. A second wet component caninclude a mixture of strengthening agents, foaming agents, and otherconventional additives, if desired. Together, the aforementioned dry andwet portions comprise an aqueous gypsum slurry that eventually forms agypsum wallboard core.

After the aqueous gypsum slurry is prepared, the slurry and otherdesired ingredients are continuously deposited to form a gypsumwallboard core (hereinafter “wallboard core” or “core”) slurry betweentwo continuously-supplied moving sheets of cover paper. The two coversheets are typically a pre-folded face paper and a backing paper. As theslurry is deposited onto the face paper, the backing paper is broughtdown atop the deposited core slurry and bonded to the prefolded edges ofthe face paper.

The whole assembly is sized for thickness utilizing a roller bar orforming plate. The deposited core slurry is then allowed to set betweenthe two cover sheets, thereby forming a board. The continuously-producedboard is cut into panels of a desired length, which arevertically-stacked, and then passed through a drying kiln wherein excesswater is removed from the board to form a strong, rigid, fire-resistantbuilding material.

The cover sheets used in the process typically are multi-ply papermanufactured from re-pulped newspapers. The face paper has an unsizedinner ply which contacts the core slurry such that gypsum crystals cangrow up to (or into) the inner ply. This gypsum crystal-paperinteraction is the principal form of bonding between the core slurry andthe cover sheet. The middle plies are sized and an outer ply is moreheavily sized and treated to control absorption of paints and scalers.The backing paper is also a similarly constructed multi-ply sheet. Bothcover sheets must have sufficient permeability to allow for water vaporto pass through during the downstream board drying step(s).

Standardized sheets (or panels) of wallboard typically are about fourfeet (about 1.22 meters) wide and about 8 feet to about 16 feet (about2.4 meters to about 4.9 meters) in length. Sheets typically areavailable in thicknesses varying in a range of about ¼ inch to about oneinch (about 0.6 centimeters to about 2.6 centimeters).

In order to provide satisfactory strength, commercially-available gypsumwallboard generally requires a density of about 1650 pounds to about1700 pounds (about 748 kilograms to about 772 kilograms) per thousandsquare feet (lbs/MSF) of one-half inch board. Because heavy orhigh-density gypsum wallboards are more costly and difficult tomanufacture, transport, store, and manually install at job sites whencompared with lighter or low-density boards, various attempts have beenmade to reduce board weight and density without sacrificing boardstrength.

Often, however, where wallboard is formulated to have a density lessthan about 1650 lbs/MSF of one-half inch board, the resulting strengthis unacceptable for commercial use.

While it is possible to formulate lighter and less dense wallboard, forexample, through the inclusion of lightweight fillers and foams into agypsum slurry, many of the lighter and less dense wallboard products areof a quality ill-suited for commercial use. It has been suggested thatreduced density wallboard of acceptable strength can be obtained byincorporating lightweight thermoplastic particles such as, for example,pre-expanded polystyrene or polyethylene latex polymer particles, intothe gypsum slurry. However, this proposed solution is deficient becauselatex polymers typically form aggregates in a high ionic strengthenvironment, such as, for example, that of a gypsum slurry.Consequently, latex polymer particles typically are not disperseduniformly throughout the gypsum product. Such a non-uniform compositionis very undesirable because it can result in local areas of weakness inthe final gypsum product.

U.S. Pat. No. 6,171,388 to Jobbins suggests that the non-uniformdistribution of latex polymer particles can be overcome by adding anexcess of nonionic surfactant(s) along with one or more latex polymersto a gypsum slurry. The inventor explains that the latex particles arebetter distributed throughout the slurry, that the resulting materialhas an improved strength to weight ratio, and that the slurry viscosityis decreased because of the added surfactant(s). Accordingly, byincreasing the amount of nonionic surfactants in the gypsum slurry, thestability of the latex particles against a high ionic environment isimproved.

However, while it may be possible to formulate gypsum slurries toinclude additional surfactants in order to confer substantial stabilityto light-weight polymeric particles against a high ionic strengthenvironment, gypsum slurries already contain a significant amount ofsurfactants and/or foaming agents (e.g., to facilitate the introductionof air bubbles into the gypsum slurry in order to reduce gypsum boardweight). The various surfactants and/or foaming agents present in thegypsum slurry may interact with one another, thereby reducing theireffectiveness, both as foaming agents and in conveying stability to thelatex polymer particles against the high ionic strength environment of agypsum slurry. Accordingly, it is desirable to avoid introducing anexcess of additional surfactants into the gypsum slurry.

In view of the foregoing, it would be desirable to produce high-strengthgypsum wallboard having weights and densities generally equal to orslightly less than those produced by conventional methods. Furthermore,such reduced weight and density boards should have a composition whichis substantially uniform throughout the gypsum product, and should meetindustry standards, having strengths similar to, or greater than,conventional wallboard. Moreover, such wallboard also should be able tobe manufactured using high-speed manufacturing apparatus and not sufferfrom other negative side-effects. For example, such high-strengthwallboard should be able to set and dry within a reasonable period oftime.

SUMMARY OF THE INVENTION

The present invention provides such compositions and methods for makinghigh-strength wallboard, preferably without introducing an excess ofadditional surfactants into the gypsum slurry.

Additionally, the present invention provides compositions and methodsfor making high-strength wallboard having substantially uniformcompositions because the tendency for latex particles to form aggregatesis substantially diminished.

Moreover, the present invention provides compositions and methods formaking high-strength gypsum wallboard having weights and densitiesgenerally equal to or slightly less than wallboard produced byconventional methods.

Furthermore, the present invention provides compositions and methods formaking high-strength wallboard using high-speed manufacturing apparatus.

One aspect of the present invention is a composition suitable for use inthe manufacture of gypsum materials, comprising calcium sulfatehemihydrate, water and a functionalized styrene butadiene latex polymer.

An additional aspect of the present invention is a gypsum wallboardpanel comprising a first cover sheet, a second cover sheet and a gypsumwallboard core disposed between said cover sheets, said core comprisingcalcium sulfate hemihydrate, water and a functionalized styrenebutadiene latex polymer.

Another aspect of the present invention is a method of making a gypsumwallboard comprising the steps of forming a slurry comprising calciumsulfate hemihydrate, water, and a functionalized styrene butadiene latexpolymer, mixing said slurry, depositing said slurry onto a cover sheet,and applying a second cover sheet atop the deposited core slurry.

Yet another aspect of the present invention is a composition suitablefor use in the manufacture of gypsum materials comprising calciumsulfate hemihydrate, water and functionalized styrene butadiene latexpolymer particles, wherein the functionalized styrene butadiene latexpolymer particles have a median diameter between about 80 nanometers andabout 220 nanometers and a gel content of about 15 percent by weight toabout 85 percent by weight.

Other advantages of the present invention may become apparent to thoseskilled in the art from a review of the following detailed description,taken in conjunction with the examples and the appended claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

According to the present invention, there are provided core compositionsincluding one or more functionalized styrene butadiene latex polymerswhich are suitable for use in the production of gypsum wallboard andother gypsum-containing products. The present invention further providesgypsum wallboard panels containing such functionalized styrene butadienelatex polymers and methods of making gypsum wallboard through the use ofthe aforementioned core compositions. Functionalized styrene butadienelatex polymers have excellent dispersion properties in high electrolyteenvironments such as the environment found in a gypsum slurry.Accordingly, the agglomeration of latex polymers that result in localareas of weakness in prior art gypsum products containing thermoplasticparticles is avoided or substantially decreased, and gypsum productscontaining a uniform distribution of light weight thermoplasticparticles are obtained.

Functionalized styrene butadiene polymers for use with the compositionsand methods of the present invention have been previously described, forexample, in U.S. Pat. No. 6,184,287 to Westerman, the disclosure ofwhich is hereby incorporated herein by reference. As described in U.S.Pat. No. 6,184,287, functionalized styrene butadiene polymers can beformed, for example, by aqueous emulsion polymerization of a monomericmixture comprising at least styrene and butadiene in the presence of aseed polymer which is prepared by aqueous emulsion polymerization ofstyrene and a salt of 2-acrylamido-2-methylpropanesulfonic acid is used.It may be possible to incorporate less of the salt of2-acrylamido-2-methylpropanesulfonic acid into the functionalizedstyrene butadiene polymers for use in accordance with the presentinvention than is contained in those styrene butadiene polymersdescribed in U.S. Pat. No. 6,184,287, as it is anticipated that theionic conditions presented in a gypsum slurry are less severe than thosepresent in gas and oil well drilling/cementing applications.

Accordingly, functionalized styrene butadiene latex polymer particleshaving high stability in high ionic strength environments are producedby modifying latex polymers, such as styrene butadiene, to include amonomer that adds ionic character to the polymer, such as, for example,a salt of 2-acrylamido-2-methylpropanesulfonic acid. In order for alatex polymer to have high stability in high ionic strengthenvironments, a latex polymer should not exhibit significant coagulationor flocculation, which can be determined visually as lumps ofagglomerated smaller latex particles, when 10 mL of a 2 percent byweight calcium chloride aqueous solution is added to 50 mL of the latexwith stirring over a time period of between 5 seconds and 30 seconds.

The content of styrene and butadiene in the functionalized styrenebutadiene polymers for use in the compositions and methods of thepresent invention can vary. Typically, the functionalized styrenebutadiene polymers contain between 20 percent by weight and 95 percentby weight styrene. Preferably, the functionalized styrene butadienepolymers contain between 45 percent by weight and 90 percent by weightstyrene. Even more preferably, the functionalized styrene butadienepolymers contain between 65 percent by weight and 85 percent by weightstyrene. Further, the functionalized styrene butadiene polymerstypically contain between 4 percent by weight and 60 percent by weightbutadiene. Preferably, the functionalized styrene butadiene polymerscontain between 7 percent by weight and 40 percent by weight butadiene.Even more preferably, the functionalized styrene butadiene polymerscontain between 10 percent by weight and 30 percent by weight butadiene.

The styrene and butadiene content of the functionalized styrenebutadiene polymers can also be described by a weight ratio of styrenecontent to butadiene content. Typically, there are between approximately10 parts by weight and approximately 1 part by weight of styrene forevery part by weight of butadiene. Preferably, there are betweenapproximately 7 parts by weight and approximately 1.5 parts by weight ofstyrene for every part by weight of butadiene. Even more preferably,there are between approximately 6 parts by weight and approximately 2parts by weight of styrene for each part of butadiene.

As set forth previously, the functionalized styrene butadiene polymersfor use with the compositions and methods of the present inventioncomprise not only styrene and butadiene, but also contain one or moremonomers that convey stability to the polymer particles against highionic strength environments, including multivalent ions. By modifyingstyrene butadiene polymers to include a monomer which conveys stabilityagainst multivalent ions, and incorporating such functionalized styrenebutadiene polymers into a gypsum slurry, reduced weight and densityboards can be advantageously manufactured. Preferably, reactive monomerswhich add anionic character to the polymer particles are incorporatedinto the styrene butadiene latex polymer particles of the presentinvention.

Preferably, monomers that addition polymerize to form a homopolymerwhich is water soluble when having a molecular weight of less than orequal to 5000 grams per mole are used to convey ionic character to thefunctionalized styrene butadiene polymers. Suitable monomers that conveyionic character typically contain at least one functionality selectedfrom the group consisting of carbonyl, carboxyl, hydroxyl, amine, amide,ether, ester, sulfate, sulfonate, sulfinate, sulfamate, phosphate,phosphonate, and phosphinate. Examples of suitable monomers that conveyionic character include 2-acrylamido-2-methyl propanesulfonic acidsalts, styrene sulfate salts, styrene sulfonate salts, allyl sulfonatesalts, 3-sulfopropyl acrylate salts, 3-sulfopropyl methacrylate salts,2-sulfoethyl acrylate salts, 2-solfoethyl methacrylate salts, maleicacid, salts of maleic acid, itaconic acid and salts of itaconic acid.The cations of these salts are normally methods such as sodium andpotassium or organic cations such as ammonium. Preferably, the cationsare sodium or potassium. The preferred monomer for conveying stabilityagainst high ionic strength environments is a salt of2-acrylamido-2-methyl propanesulfonic acid. Most preferably, the sodiumsalt of 2-acrylamido-2-methyl propanesulfonic acid is incorporated intothe functionalized styrene butadiene polymers of the present invention.

The content of the monomer(s) that conveys ionic character to thestyrene butadiene latex polymers for use in the compositions and methodsof the present invention is typically between about 0.25 percent byweight and about 20 percent by weight of the functionalized styrenebutadiene polymer. Preferably, the functionalized styrene butadienepolymer will include between about 0.5 percent by weight and about 10percent by weight of monomer that conveys ionic character. Morepreferably, the functionalized styrene butadiene polymer will includebetween about 1 percent by weight and about 5 percent by weight ofmonomer that conveys ionic character.

In addition to styrene, butadiene, and one or more monomer that conveysionic character, the styrene butadiene polymers of the present inventionmay also include about 0.25 percent by weight to about 20 percent byweight of one or more comonomers selected from the group consisting ofhydroxyethyl acrylate, hydroxyethyl methacrylate, acrylonitrile,methacrylonitrile, acrylamide and methacrylamide. Functionalized styrenebutadiene polymers in accordance with the present invention whichinclude one or more of those comonomers exhibit enhanced stabilityagainst high ionic strength environments including multivalent ions.More typically, the polymers of the present invention include about 0.5percent by weight to about 10 percent by weight of one or more of suchcomonomers. Even more typically, the polymers of the present inventioninclude about 1 percent by weight to about 4 percent by weight of one ormore such comonomers.

Further, the functionalized styrene butadiene polymers in accordancewith the present invention may include other addition monomers. Examplesof such addition monomers include isoprene, chloroprene,alpha-methylstyrene, 4-methylstyrene, 4-tert-butylstyrene,4-ethylstyrene, divinylbenzene, vinylidene chloride, 2-vinylpyridine,4-vinylpyridine, acrylic acid (including metal and ammonium saltsthereof), methacrylic acid (including metal and ammonium salts thereof),substituted and unsubstituted amides (aside from acrylamide andmethacrylamide which have been previously described herein as beingcomonomers that enhance the stability of the styrene butadiene polymersin accordance with the present invention against multivalent ions),nitriles, and C₁ to C₁₂ esters. The polymers in accordance with thepresent invention should not contain more than about 30 percent byweight of one or more of the listed addition monomers. More typically,the polymers in accordance with the present invention contain less thanabout 15 percent by weight of addition monomer(s).

For example, styrene butadiene polymers which are particularly useful inthe methods and compositions according to the present invention mayinclude approximately 2 percent by weight to approximately 6 percent byweight, preferably approximately 2.5 percent by weight to approximately5.5 percent by weight, of the sodium salt of2-acrylamido-2-methylpropanesolfonic acid, approximately 65 percent byweight to approximately 80 percent by weight, preferably approximately67.2 percent by weight to approximately 77.5 percent by weight, ofstyrene, approximately 15 percent by weight to approximately 30 percentby weight, preferably approximately 15 percent by weight toapproximately 26 percent by weight, of butadiene, approximately 0percent by weight to approximately 3 percent by weight of hydroxyethylacetate, and approximately 0 percent by weight to approximately 10percent acrylonitrile.

Typically, the functionalized styrene butadiene polymers in accordancewith the present invention are prepared via well known aqueous emulsionpolymerization methods. Styrene, butadiene, one or more monomers thatconveys ionic character to the polymer and, optionally, one or morecomonomers and one or more addition monomers, are generally polymerizedin the presence of water, free radical initiators, emulsifyingsurfactants and chelating agents. Preferably, a non-ionic surfactant oran anionic surfactant is used to emulsify the monomer constituents.Additional ingredients including chain transfer agents, biocides,defoamers and antioxidants may also be added to the mixture.

Upon activation of the free radial initiator, the constituent monomerspolymerize to form a polymeric product in accordance with the presentinvention. To a certain extent, the arrangement of the constituentmonomers in the final product can be controlled through the order inwhich the monomers are added to the reaction mixture. When all of theconstituent monomers are added simultaneously, the product will have amore random distribution of monomers than when the constituent monomersare added sequentially or in stages.

The amount of functionalized styrene butadiene polymer that isincorporated into a gypsum slurry can vary a great deal, and nearly anyamount can be added to a gypsum slurry. Preferably, one or morefunctionalized styrene butadiene polymers are added to a gypsum slurryin a total amount from about 0.1 percent by weight to about 10 percentby weight based on the weight of calcium sulfate hemihydrate. Morepreferably, one or more functionalized styrene butadiene polymers areadded to a gypsum slurry in an amount from about 0.25 percent by weightto about 5 percent by weight based on the weight of calcium sulfatehemihydrate. Most preferably, one or more functionalized styrenebutadiene polymers are added to a gypsum slurry in an amount from about0.25 percent by weight to about 5 percent by weight based on the weightof calcium sulfate hemihydrate. Most preferably, one or morefunctionalized styrene butadiene polymers are added to a gypsum slurryin an amount from about 0.5 percent by weight to about 1.5 percent byweight based on the weight of calcium sulfate hemihydrate.

Typically, the polymeric chains that are formed are further reacted orcrosslinked by heat, light, ultraviolet radiation, or other reactantsand/or catalysts to form a gel which has significantly increasedstrength with respect to the individual polymer chains. As used herein,the term “gel content” refers to the proportion of the polymer chains ofthe polymer particles that have been crosslinked, thereby constituting apart of the gel network. Preferably, the styrene butadiene latexparticles for use in the present invention have a gel content from about15 percent to about 85 percent. More preferably, and to achieve the fulladvantage of that present invention, the styrene butadiene latexparticles have a gel content from about 30 percent to about 70 percent.Most preferably, the styrene butadiene latex particles have a gelcontent of about 50 percent.

The gel content of the functionalized styrene butadiene polymerparticles can be quantitatively measured. For example, by continuouslyextracting (e.g., by soxhlet extracting) the reaction product aftercrosslinking processing is complete, the weight of the crosslinkedpolymer material can be obtained. A continuous extraction method allowspolymers which are soluble to be removed from the mass of crosslinkedpolymer which typically is not soluble in most or any solvents.Accordingly, the use of a solvent in which a polymer is soluble, and inwhich the crosslinked polymer is insoluble, is necessary for thesuccessful implementation of this procedure. By dividing the weight ofthe crosslinked polymer material by the total weight of the materialthat was continuously extracted, and multiplying by 100, the gel contentvalue may be obtained. For example, 100 grams of a thermoplastic polymerwhich is soluble in chloroform can be crosslinked with ultravioletradiation. The gel product is placed in a soxhlet extractor and washedrepeatedly with hot chloroform. Once the extraction is complete, the gelis dried and weighed. If the mass of the gel is 90 grams, then the gelcontent is 90% (the other 10% mass was unreacted polymer dissolved bythe chloroform). The degree of crosslinking can be regulated by thetime, intensity, and concentration of the polymer treatment.

In another embodiment according to the present invention, the monomersare copolymerized with and crosslinked by one or more polyacrylates.Preferably, dimethacrylates having from 2 to 30 ethoxy units betweeneach methacrylate functionality are used. More preferably,dimethacrylates having at least four ethoxy units between eachmethacrylate functionality are used, and most preferably,dimethacrylates having from four to 22 ethoxy units between methacrylatefunctionalities are utilized. The incorporation of such flexiblecrosslinking agents allows for the production of materials that canwithstand substantial compressive forces, without incurring damage tothe gypsum crystals of the gypsum-containing product.

The functionalized styrene butadiene polymers for use with the methodsand compositions of the present invention typically have a medianparticle diameter between about 80 nanometers and about 220 nanometers.More preferably, the median particle diameter is between about 140 andabout 200 nanometers. Most preferably, the median particle diameter isbetween about 170 and about 190 nanometers. The median particle diameterof the functionalized styrene butadiene polymers can be determined byknown methods including electron microscopy techniques. Furthermore, theparticle diameter can be varied by using techniques known by those ofordinary skill in the art, for example, by varying the concentration ofemulsifying surfactant(s) in the polymerization reaction. Greaterconcentrations of emulsifying surfactant(s) result in increased particlenumber and decreased particle diameter. Surfactants include sodiumstearate, sodium laurate, sodium palmitate, potassium stearate,potassium laurate, potassium palmitate, sulfates (e.g. sodium laurylsulfate), and sulfonates (e.g. sodium dodecylbenzene sulfonate).Non-ionic surfactants such as poly(ethylene oxide), poly(vinyl alcohol)and hydroxyethyl cellulose are sometimes used in conjunction with othersurfactants. Surfactants are generally used at about 0.2 weight % toabout −3 weight % based on the amount of water/solvent.

A preferred method for manufacturing the core composition and wallboardof the present invention initially includes the premixing of dryingredients in a mixing apparatus. The dry ingredients can includecalcium sulfate hemihydrate (stucco), an accelerator, and anantidesiccant (e.g., starch), as described below in greater detail.

The dry ingredients are mixed together with a “wet” (aqueous) portion ofthe core composition in a pin mixer apparatus. The wet portion caninclude a first component (referred to as a “paper pulp solution”) thatincludes a mixture of water, paper pulp, and optionally,fluidity-increasing agents. A set retarder can also be included. Thepaper pulp solution provides a major portion of the water that forms thegypsum slurry of the core compositions. A second wet componentpreferably includes a mixture of functionalized styrene butadiene latexpolymer, strengthening agent(s), foaming agent(s), and otherconventional additives, if desired. It should be noted that thefunctionalized polymeric materials are typically supplied as wateremulsions to facilitate ease of incorporation into the gypsum slurry,and can also be added directly into the mixing apparatus or can beincorporated into the pulp solution prior to addition to the mixingapparatus.

The produced core composition slurry is deposited between paper coversheets to form a sandwich. The core composition is allowed to cure orset, whereby calcium sulfate hemihydrate is converted to calcium sulfatedihydrate. The product is then preferably dried to remove any excesswater not consumed in the reaction forming the calcium sulfate dihydrate(Excess water is preferably included to decrease the viscosity of theslurry during production.)

The setting reaction produces gypsum crystals, which are interwoven tocontribute strength to the wallboard core. The resultingcrystal-to-crystal interaction is important to the final strength of thegypsum wallboard product.

The gypsum crystals also preferably interlock with paper fibersprotruding from the surface or cover papers, thus bonding the papers tothe core. This bonding or interaction also increases the strength of thewallboard product. The compositions of the present invention alsopreferably are able to produce wallboards having increased paper-to-corebonding.

The method of the invention allows a substantial reduction in boardweight and density, while producing wallboard that can meet industrystrength standards. By way of example only, the methods of the presentinvention can allow for the production of high strength wallboardweighing about 1550 pounds per thousand square feet to 1650 pounds perthousand square feet (based on one-half inch thick board), and evenlower in some cases.

The preferred ingredients of the wallboard core composition of thepresent invention will now be described in more detail. The firstingredient of the wallboard core composition of the present invention iscalcium sulfate hemihydrate, or stucco (CaSO₄.½H₂O). Calcium sulfatehemihydrate can be produced by the methods described above. Calciumsulfate is described, for example, by Petersen, D. J., et al. “CalciumCompounds (Calcium Sulfate), “Kirk-Othmer Encyclopedia of ChemicalTechnology, (1992 4^(th) ed.), vol. 4, pp. 812-26.

As is known by those skilled in the art, there are two types of calciumsulfate hemihydrate, the α-hemihydrate form and the β-hemihydrate form.These two forms are typically produced by different types of calcinationprocesses and differed structurally to some extent. Either type ofcalcium sulfate hemihydrate is suitable for use with the presentinvention.

Other dry ingredients are preferably included in the core composition,including an accelerator which can be used to control, within certainlimits, the crystal growth rate and set time of the stucco. Examples ofsuitable accelerators include ball mill accelerators (“BMA”) andpotassium sulfate, although many others are known to those skilled inthe art. In some cases, the presence of the functionalized styrenebutadiene polymer in accordance with the present invention may requireincreased amounts of accelerator, e.g., 0.04 percent by weight to 0.50percent by weight, based on the weight of calcium sulfate hemihydrate.

An antidessicant such as starch is also included in order to prevent thedehydration of calcium sulfate dihydrate crystals formed during settingof the core composition. In some products, additional lightweightaggregates (e.g., expanded perlite or vermiculite) can be included inthe dry ingredients.

An aqueous slurry or solution of paper pulp is also included in the corecomposition. The pulp solution comprises water and paper fibers (“paperpulp”), and may also include a retarder, corn starch and/or potash. Theretarder is used in conjunction with the aforementioned accelerator inorder to tailor the set time of the core composition. Retarding agentsare typically used with the present invention at very low rates (if atall), for example at about 0.0007 percent by weight, based on the weightof the core composition.

The paper pulp solution can also include one or more of a number ofadditives that increase the fluidity of the slurry and/or reduce thewater requirements of slurry. Materials used as fluidity-enhancingand/or water-reducing agents include “lignosulfonates” which areavailable commercially either in liquid or powder form. Agents suppliedin liquid form can be either incorporated in the pulp solution or addeddirectly to the mixing operation.

The pulp solution can be prepared by blending or mixing the aboveingredients with water in a blending apparatus. Alternatively, aconcentrated pulp solution using only a small volume of water can beproduced. In this case, the remainder of the core mix water requirementis made up with a separate water source. An excess of water with respectto the above-described rehydration reaction is preferably included inorder to provide satisfactory flowability of the core composition.Typically, about 75 weight parts water are used per 100 weight partsstucco. Preferably, high shear mixing “pulps” the material, forming ahomogenous solution or slurry. The pulp solution can be transferred to aholding vessel, from which it can be continuously added to the corecomposition mix. The paper fibers in the pulp solution serve to enhancethe flexibility of the gypsum wallboard. Gypsum wallboard made withoutfibers is typically very brittle and more susceptible to breakage duringhandling. The paper fibers also aid in evenness of drying duringmanufacture, as well as enhance the ability of the final wallboardproduct to accept and hold nails during installation.

As indicated above, the wet portion of the core composition alsopreferably includes a component that incorporates foam into the slurry,a strength-enhancing agent, and functionalized styrene butadiene latexpolymer. Foam introduces air voids into the core though the use of afoam that contains very little solid material, but is resilient enoughto resist substantial breakdown in the mixing operation. In this manner,the density of the core can be controlled. Known foaming agents may besupplied in either liquid or flake (powdered) form, and may be producedfrom soaps known in the art. Strengthening agents in the form of anacrylic polymer emulsion suitable for use in the present invention aredisclosed in U.S. Pat. No. 5,879,825, the disclosure of which is herebyincorporated herein by reference.

Furthermore, gypsum wallboard can be adapted for wet and exteriorapplications by incorporating various materials into the core to impartincreased water absorption resistance to the board. Useful materialsinclude silicone water repellents, waxes, and asphalt emulsions. Thesematerials are typically supplied as water emulsions to facilitate easeof incorporation into the board core, and can be added directly into themixing apparatus or incorporated into the pulp solution prior toaddition to the mixing apparatus. The foregoing detailed description isgiven for clearness of understanding only, and no unnecessarylimitations should be understood therefrom, as modifications within thescope of the present invention will be apparent to those of ordinaryskill in the art.

1. A method of making a gypsum wallboard, the method comprising: forminga slurry comprising calcium sulfate hemihydrate, water, and afunctionalized styrene butadiene latex polymer; crosslinked bydimethacrylate having from 2 to 30 ethoxy units between methacrylatefunctionalities mixing the slurry; depositing the slurry onto a firstcover sheet; and applying a second cover sheet atop the depositedslurry.
 2. The method of claim 1, wherein the functionalized styrenebutadiene latex polymer includes at least about 0.25 percent by weightof a monomer that conveys ionic character to the functionalized styrenebutadiene latex polymer.
 3. The method of claim 2, wherein the monomeris selected from the group consisting of 2-acrylamido -2-methylpropanesulfonic acid salt, styrene sulfate salt, styrene sulfonate salt,allyl sulfonate salt, 3-sulfopropyl acrylate salt, 3-sulfopropylmethacrylate salt, 2-sulfoethyl acrylate salt, 2-sulfoethyl methacrylatesalt, maleic acid, salts of maleic acid, itaconic acid, salts ofitaconic acid, and mixtures thereof.
 4. The method of claim 2, whereinthe monomer is a metal salt of 2-acrylamido-2-methyl propanesulfonicacid.
 5. The method of claim 2, wherein the monomer is a sodium salt of2-acrylamido-2-methyl propanesulfonic acid.
 6. The method of claim 1,wherein the functionalized styrene butadiene latex polymer is preparedby aqueous emulsion polymerization of a monomeric mixture comprisingstyrene and butadiene in the presence of a seed polymer prepared byaqueous polymerization of styrene and a salt of 2-acrylamido-2-methylpropanesulfonic acid.
 7. The method of claim 1, wherein the medianparticle diameter of the functionalized styrene butadiene latex polymeris about 170 nanometers to about 190 nanometers.
 8. The method of claim1, wherein the styrene butadiene latex polymer has a gel content ofabout 50%.